1. Field of the Invention
This invention concerns the use of N-aminopropyl derivatives of 2,4-diamino-2-methylpentane as epoxy curing agents.
2. Description of the Related Art
Epoxy resins constitute a broad class of polymeric materials having a wide range of physical characteristics. The resins are characterized by epoxide groups which are cured by reaction with certain catalysts or curing agents to provide cured epoxy resin compositions with certain useful properties. Conventional curing agents include such compounds as polyamines, polycarboxylic acids, anhydrides and Lewis acids.
The use of diamines as epoxy curing agents has long been known in the art. British Pat. No. 680,997 is one of numerous disclosures of the use of a diamine to cure resinous compositions. In particular, diacetone diamine (2,4-diamino-2-methylpentane) is disolosed as a preferred diamine for epoxy curing.
Appicants have verified that this diamine is very useful for curing epoxy resins where slow curing times are required. However, the user of 2,4-diamino-2-methylpentane suffers the hazards imposed by its toxicity. Toxicity tests have shown that 2,4-diamino-2-methylpentane is extremely irritating to the eyes and skin, and oral ingestion tests show an LD.sub.50 of 0.431 g/kilogram, classifying it as a toxic material. Applicants have no toxicity data on N-aminopropyl derivatives of 2,4-diamino-2-methylpentane. However, our data show the N-aminopropyl derivatives to be less volatile than 2,4-diamino-2-methylpentane. This material should, however, first be tested for toxicity by any user.
U.S. Pat. No. 2,486,648 to Haury describes a method for preparing diamines such as 2,4-diamino-2-methylpentane from pyrimidines. In particular, Haury describes experimentally the reduction of the tetrahydropyrimidine shown below. ##STR1## Many other derivatives of tetrahydropyrimidines are proposed for study, including, in a very broad sense, the derivatives used by applicants to cure epoxy resins. Haury makes no mention of the use of his derivatives as epoxy curing agents. An example of a substituted pyrimidine ring similar to the type Haury describes as useful for the preparation of diamines is found in U.S. Pat. No. 4,391,957, FIG. V of Drawing I.
Other derivatives of 2,4-diamino-2-methylpentane are also known. U.S. Pat. No. 4,537,601 mentions that N-isopropyl-2,4-diamino-2-methylpentane is useful for reacting with a maleic group to form an imide in the preparation of antistatic compositions.
Applicants' invention is the use of N-aminopropyl derivatives of 2,4-diamino-2-methylpentane as epoxy curing agents, and the resulting epoxy resin compositions. Among other improvements over the use of 2,4-diamino-2-methylpentane as an epoxy curing agent, it was found that N-aminopropyl derivatives of this diamine substantially reduce the gel time.